In some areas, selective catalytic reduction (SCR) units may be installed in the
flue to reduce NO x . These systems employ a catalyst matrix with very low
pressure drop (inches of water). Ammonia is injected into the flue ahead of the
catalyst through a grid of nozzles. The ammonia is vaporized outside the flue and
diluted into hot air to improve distribution. The SCR must be located in an exact
temperature range (depending on the catalyst chosen). Sometimes, these are
retrofitted to existing hydrogen plants, with the convection coils rearranged to
ensure the correct temperature range.
• Furnace draft control – SMR furnaces may be natural draft, induced draft, or
forced/induced draft. You seldom see forced draft-only furnaces in this service.
Older units tend to be natural draft. For down-fired furnaces or where an SCR is
required, induced draft is used. If a furnace has air preheat, it will generally have
both forced and induced draft fans.
The burner registers provide additional draft control. The registers must be open
enough to allow the control system to work.
PSA off-gas combustion introduces a swing into the fuel gas controls that will
affect draft. The draft will vary slightly every couple of minutes as the PSA
cycles. Controls around the PSA attempt to moderate the swing, but generally
cannot eliminate it. With careful tuning of controllers, the swing can generally
be managed. Keep in mind that swinging draft also means swinging excess
oxygen levels.
• SMR process outlet header – The high process outlet header temperatures from
an SMR furnace give rise to special problems when transferring the effluent to
the waste heat boiler. The outlet headers inside the firebox are generally high
alloy (e.g., Incoloy). The high alloy is transitioned to a lower alloy header,
normally something like 1¼ Cr steel, that is lined with a refractory system.
Figure 15 illustrates the types of refractory arrangements normally seen in this
service.
Inconel
Shroud
3" Hot Facing
Refractory
Dual Refractory
Lined Pipe
Outer Shell
1 1 / 4 Cr Steel
4-6" Insulating
Refractory
Shrouded Refractory
Lined Pipe
Op
Open
Pipe
Open
Pipe
Fig. 15 Typical refractory systems for SMR furnace process outlet
Hydrogen Production and Management for Petroleum Processing
597
flue to reduce NO x . These systems employ a catalyst matrix with very low
pressure drop (inches of water). Ammonia is injected into the flue ahead of the
catalyst through a grid of nozzles. The ammonia is vaporized outside the flue and
diluted into hot air to improve distribution. The SCR must be located in an exact
temperature range (depending on the catalyst chosen). Sometimes, these are
retrofitted to existing hydrogen plants, with the convection coils rearranged to
ensure the correct temperature range.
• Furnace draft control – SMR furnaces may be natural draft, induced draft, or
forced/induced draft. You seldom see forced draft-only furnaces in this service.
Older units tend to be natural draft. For down-fired furnaces or where an SCR is
required, induced draft is used. If a furnace has air preheat, it will generally have
both forced and induced draft fans.
The burner registers provide additional draft control. The registers must be open
enough to allow the control system to work.
PSA off-gas combustion introduces a swing into the fuel gas controls that will
affect draft. The draft will vary slightly every couple of minutes as the PSA
cycles. Controls around the PSA attempt to moderate the swing, but generally
cannot eliminate it. With careful tuning of controllers, the swing can generally
be managed. Keep in mind that swinging draft also means swinging excess
oxygen levels.
• SMR process outlet header – The high process outlet header temperatures from
an SMR furnace give rise to special problems when transferring the effluent to
the waste heat boiler. The outlet headers inside the firebox are generally high
alloy (e.g., Incoloy). The high alloy is transitioned to a lower alloy header,
normally something like 1¼ Cr steel, that is lined with a refractory system.
Figure 15 illustrates the types of refractory arrangements normally seen in this
service.
Inconel
Shroud
3" Hot Facing
Refractory
Dual Refractory
Lined Pipe
Outer Shell
1 1 / 4 Cr Steel
4-6" Insulating
Refractory
Shrouded Refractory
Lined Pipe
Op
Open
Pipe
Open
Pipe
Fig. 15 Typical refractory systems for SMR furnace process outlet
Hydrogen Production and Management for Petroleum Processing
597